bevy_text: give swash a stable font id

[kristoff3r]

Objective

Disclaimer: AI was used to find/triage the issue, but I spent quite a bit of time verifying everything. I am not a text rendering exptert though.

While investigating the performance regression fixed in #24663 in my own project, I stumbled upon another one related to caching and font hinting.

As mentioned in https://docs.rs/swash/latest/swash/struct.FontRef.html#owning-your-fonts, using the FontRef constructors implicitly generates new CacheKey instances, which breaks the internal caching layer. Using the scale builder without giving an id does this internally, which is a bit of a footgun. I believe cosmic_text handled this before, and that this is a regression from the move to parley.

When running many_text2d --hinting the startup time went from 7s to 500ms. I couldn't get a good tracy screenshot running with --recompute because the fps on main is too low. Note that the measurements are based on main before #24663.

Solution

Give the builder a stable font id. Someone should confirm if this is indeed a valid ID to give to swash, or if I forgot to take some feature(s) into account.

I also added a hinting flag to many_text2d that I used for testing, seems useful to keep.

Testing

Ran many_text2d, saw better performance and that the text looked similar.

[alice-i-cecile]

Okay, I've dug into the code here, and I think this fix is correct. What a nasty footgun.

However, there's another usage of this pattern that needs to be fixed, and should be done at the same time, over in

bevy/crates/bevy_ui/src/widget/text_input_layout.rs

Line 363 in 4b0579b

.builder(font_ref)

[kristoff3r]

Okay, I've dug into the code here, and I think this fix is correct. What a nasty footgun.

However, there's another usage of this pattern that needs to be fixed, and should be done at the same time, over in

bevy/crates/bevy_ui/src/widget/text_input_layout.rs

Line 363 in 4b0579b

.builder(font_ref)

I fixed that one as well, and tested with the text_input example.

[ickshonpe]

Yep this is correct, it's a nice improvement.